1. Field of the Invention
The invention is in the field of sonic well logging and relates particularly to a method of filtering sonic log data to provide more accurate identification of the arrival and slowness of such sonic wave components as casing, compressional, shear and Stoneley.
2. Description of the Prior Art
Sonic well logging is used to provide surveys for formations traversed by boreholes. Many techniques and tools for carrying out sonic well logging are extensively described in the literature. In particular, measurements are made of the velocities of sonic waves to reveal valuable information concerning the types of rocks and the porosity of the rocks comprising the formations surrounding the borehole. The velocities are more commonly expressed as slowness, a sonic parameter, measured in micro-seconds per foot.
Under ideal conditions, the identification of the various components comprising the sonic wave may be accomplished with ease. In the classic textbook case, the first arriving wave in a cased borehole is the casing wave, the second is the compressional wave, followed by the shear wave and finally the Stoneley wave. In practice, however, the determination of slowness is complicated by a number of factors. For example, at extreme depth, the compressional wave may arrive ahead of the casing wave and the shear wave arrival can be difficult to detect because it propagates more slowly and its arrival under field conditions will typically be obscured by compressional energy and other acoustic modes inside the bore hole. The determination, therefore, of shear slowness directly from arrival time is at best difficult and at times impossible.
There are two major approaches in processing sonic data to obtain information such as propagation velocity, energy, and attenuation of the individual wave components such as casing, compressional, shear and Stoneley waves from the overall wave train. The first is a model base approach. This approach assumes a relationship between various wave components. The signal processing technique is very efficient and the outputs are specific. One example of the model base approach is a direct phase determination method which assumes that the compressional wave is the wave component that arrives first, and the shear wave is related to the compressional wave via the Poisson's ratio relationship. However, if the wave train components arrive in different orders than assumed, or if indeed the wave components overlap, erroneous results are obtained.
The second approach does not assume any initial relationship between the various components. The signal processing technique gives rise to data that is more comprehensive in the sense that the results are less specific, including both correct and erroneous information. However, the important factor is that the results do contain representations of the correct parameters sought to be measured. One such technique, the slowness time coherence method (STC Method) is an example of this approach. The method is described in copending application filed by co-employees Christopher V. Kimball and Thomas L. Marzetta, entitled "Sonic Well Logging" now U.S. Pat. No. 4,594,691, executed on Dec. 29, 1981 and assigned to the same assignee as the present application.
In the Kimball et al application, the method includes the production of sonic logs which represent the receipt, at successive borehole depth intervals or levels, of sonic signals by receivers which are spaced along the length of a borehole tool from each other and from at least one transmitter of the sonic signals. These sonic logs are analyzed to determine whether they include an arrival of sonic energy with a time T and the slowness S for all (T, S) combinations which have been found to be reasonable from past experience. The measure of interest is called a coherence measure and is designated R.sup.2 (T, S). It corresponds to a measure sometimes called "Semblance" in seismic work. For each given depth level z in the borehole, the surface made up of the determined measures of coherence for (T, S) combinations tends to have peaks corresponding to the different components of the sonic signals arriving at the receivers, for example, the casing, the compressional, the shear and the Stoneley components. It has been discovered that the arrival time and slowness associated with these peaks have unexpectedly significant relationship to the subsurface formation and that, accordingly, new logs associated with parameters of those peaks can be produced which give significant clues to the subsurface formation. Because the STC method is "robust," it gives rise to peaks that are representative of the characteristics sought to be measured as well as peaks which are erroneous representation of the characteristics. Accordingly, the results of the STC method require a post-processing procedure to interpret the peaks and thereby produce accurate logs of slowness, energies, and other characteristics for the casing, compressional, shear and Stoneley wave components. Such a post-processing procedure is provided in accordance with the present invention.